If you're on the fence about the E5c's... read this.

Let me start off this review by saying that I am not normally one to gush about a product unless it is truly spectacular. Generally, even my positive reviews are tempered with sober observations concerning drawbacks, however small they may be. Having said that, I can't find a single thing to complain about the Shure E5c earbuds except for the painfully obvious fact that they cost darn-near $500.00. I can, however do my best to give a first hand impression of the different features of the product. By the way, I started off with the E2c's (which were great) and I was so impressed I returned them so I could see if the E5c's could possibly be better. They are.

Sound
Why save the best for last? Obviously, if you're contemplating a set of $500 earbuds, you are a serious audiophile... that or you're trying to unload a whole bunch of cash. (If the latter is the case, let me know... I'm sure I could take some off your hands). Assuming the prior, here's the bad news: you have never heard your music before the way it should be heard. No, seriously. As cheesy as it sounds, there's just no denying the fact that I'm hearing stuff (good and bad) that I've never noticed in years of listening to the same songs. What's good? Well, first and foremost, you just hear more. I don't know how to describe it but I know it's not my imagination because I've conducted experiments using my fiance, friends and family as test subjects. All of them agree that there is more to the music when you hear it through E5c's. I imagine this means that the speakers in the headphones are extremely sensitive and thus reproduce more sounds than my other speakers/headphones. As an example, when you're listening to Pink Floyd's Another Brick in the Wall, there are voices in the background that I had never heard before. I was so shocked, I actually turned around to see if someone was behind me having a conversation (not that I would have heard it... these earbuds are almost sound-proof).
What's bad? Well, here's a drawback... songs with poor mixing or older sound effects are exposed for all their flaws. You can't get anything by these earphones. Again, turning to Pink Floyd, in Dark Side of the Moon there is a phrase where a human voice turns into a guitar wailing. I've never noticed the transition before, but with these speakers, it's as obvious as a motorcycle changing gears.
Overall, the sound on these headphones is superior to anything I've ever heard.

Sound Isolation:
The E5c is not a sound canceling device. It falls into a category of products called sound isolators. What that means is that they basically, in addition to being in-ear speakers, function as ear plugs. In this capacity, the E5c excels. Using the triple-flange sleeves you are deaf to the world. The only drawback to the triple-flange sleeves is that they do get uncomfortable after a while. Fortunately, there are three (yes, three) other options, each in different sizes that come with the phones: flex sleeves, soft flex sleeves, and foam. The flex sleeves are made out of rubber and each comes in three sizes (S,M,L). Naturally, the soft sleeves are the most comfortable but they do offer the least in the way of sound isolation... ahh drawbacks. The regular sleeves are a nice happy-medium, but I still prefer the triple-flange sleeves. I have not tried the foam sleeves, but they work just like those foam ear plugs that you compress and then let expand in your ears. All the sleeves are replaceable through Shure.
Perhaps I should have said so earlier, but it seems pretty obvious... the sleeves are all interchangeable. They just slide on and off.

Construction:
I have no issues with the construction, save to say that the E5c's feel really, really bomber. The case is cool too.

Fit:
I have already discussed the inner-ear fit so if you want to read about that, read the part on sound isolation. However, one addition is that the E5c supports specially made ear inserts. (Meaning, you can pay someone $200 to make a mold of your ear and use that mold to make a special sleeve). As for the rest of the fit, the wires are meant to loop over your ears and either down your front or back. I go down the front. To help keep the wires in place, Shure uses a special "memory wire" (my made-up term) for the length of wire that loops over your ear. Once you mold it around your ear, it more or less holds that shape. It takes a bit of work to get it just right, but after that, you forget you're even wearing anything.

Now... to respond to some other reviews I've read about the E5c: the driver is not uncomfortable at all for me. As you have no doubt seen, the driver is sizable and is designed to fill your whole outer ear. Some people claim that it is horribly uncomfortable for them. Maybe so, but I have pretty small ears and it fits just fine for me.

Overall:
5 out of 5 stars. If you can afford it, and you are slightly fanatical about hearing your music, then get the E5c's. If you're just looking to get rid of money, drop me a line.

What follows is information from the manufacturer's site and some helpful links:
Sound isolating and unobtrusive Shure earphones are designed specifically for use with Shure PSM systems and deliver the incredible sound and performance that ear buds and other headphones simply can't. Don't go In Ear without them.

Along with their high fidelity, full-bandwidth transducers, the natural design of these stereo earphones allows them to rest comfortably in the ear, providing a tight seal for sound isolation and a snug, secure fit. Shure Sound Isolating Earphones can be used with a variety of foam, flex, or triple flange accessory sleeves to get a personal fit tailored to your ears. The universal 3.5mm (1/8 in.) plug connects to Shure's PSM body pack receivers, as well as many other personal audio applications where high fidelity is deemed appropriate.

E5 Sound Isolating Earphones
Dual high energy micro-speakers with wide-frequency stereo sound, great accuracy and isolation from outside noise.

Specifications:

Transducer Type: dual low mass/high energy
Sensitivity (at 1 kHz): 122 dB SPL/mW
Impedance (at 1 kHz) 110 ohms
Output Connector: Gold-plated stereo, 3.5 mm (1/8-inch) phone plug
Cable Length: 1.55 m (61 in.)

Comparison:
http://www.shure.com/selectionguides/sel-earphones.asp
http://www.shurestore.com/earphones/eseries_comparison.html

Users Guide (.pdf):
http://www.shure.com/pdf/userguides/guides_psm/earphones_en.pdf